Adiabatic—a what?

If you listen to the explanations given by weather forecasters, you may have heard the word adiabatic.

No, it’s not a disease related to blood sugat—that has a BET in the middle, not a BAT.

Instead, it is one very cool reason that storms suddenly pop up out of a clear sky in the desert.

Adiabatic cooling or warming happens when the pressure on a blob of air increases or decreases. When desert air hits a mountainside and rushes up the mountain, the air expands, gets “thinner” and cools as it rises to a higher elevation.

This means that if there is any moisture in the air, there is less energy to keep it suspended, so the moisture is more likely to condense, and form rain.

The opposite happens when air flows downslope—it warms as the pressure increases because the molecules are closer together and bump into each other more.

Maybe that is one reason that places east of mountains in North America are often more warm and dry than places to the west of mountains. Not only does the moisture get wrung out of the air on the western upslope, even very wet air from the ocean, but following that, some of the moisture on the downslope gets absorbed back into the falling, compressing, warming air.

Cool!

Or rather, cool, then warmer and drier!

 

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Limiting factors in the desert

In some ways the term “limiting factor” is almost a definition of deserts, because deserts lack one or more essential ingredients for the survival of most kinds of plants and animals.

First, a desert is, by definition, a place with very little available water, either because there is little rainfall, or because it evaporates so fast. It may also be that most of the precipitation is unavailable because it is frozen most of the year, as in the far north or south, or in some high altitude regions.

Some areas may get bursts of moisture, even flooding, but it is episodic and not dependably available most of the year.

Temperature is another limiting factor. Many deserts get too hot for most organisms. Some deserts near the north and south poles are too cold. And yet others have wide swings in temperature.

Wind is another. There are few trees or shrubs to stop the wind. Desert mountains, river-courses or other sheltered areas may harbor small oases in their wind and sun shadows.

One unusual limiting factor is lack of sunlight. This can occur in deserts near the poles.

Especially interesting are the adaptations that plants and animals have developed to deal with these limiting factors. Many of them are what we talk about here.

Why does the cactus look like that?

The first thing you notice about a cactus is generally the spines. Some have short, nasty little almost invisible fishhooks, some inches-long needles. Then there’s the form of the plant. Most cacti are plump, rounded, often stubby. They tend to be grayish-green in color. And they usually do not have obvious leaves.

Why?

Whenever you come across questions like this, think about the ecosystem where the organism lives. Cacti live in dry climates with a lot of sun, and few other plants, especially leafy ones.

So keeping cool is an issue. A light neutral color does not absorb a huge amount of the sun’s heat. There are very few plants with dark bark, or skins, or leaves in the desert.

Retaining moisture is perhaps the most important issue for a desert plant. The blunt, compact shape of cacti, their thick skins, and lack of leaves are all moisture-conserving features.

And lastly, toothsome, moist leaves would attract all kinds of herbivores. So the lack of leaves discourages plant-eaters, and the big spines and tough hide protect any soft, moist tissue a cactus does have.

Dry rain

Even the simplest things about the desert can be strange.

Rain, for instance.

When you need rain the most, where you need it most, seems to be the place you find it least.

And sometimes it comes unusually close, but never arrives.

An eerie and frustrating example is the dry rain that can occur in the desert west, called “virga.”

In the distance, you will see the clouds form, then curtains of rain finally begin to fall. But the rain disappears in the middle of the sky, between cloud and land.

Leaving you high and dry. Or low and dry.

Virga actually does happen in other regions, and can involve snow as well as rain.

It’s a sight to see, watching the rain come down, but never land on land.